The invention relates to a device for low power loss control and flow control for hydraulic machines particularly for hydraulic cylinders, which are fed from a reservoir with constant or semi constant high pressure. Such a situation for instance occurs in case of a so-called free piston engine which pumps up hydraulic medium from a pressure accumulator with low pressure of for instance 10 bar to a pressure accumulator with high pressure of for instance 400 bar, in which the hydraulic machine is connected between the low and high pressure tank.
In many cases the hydraulic machine is connected through a throttle valve. With a low load of the machine and as a result a low necessary pressure difference over the machine the pressure difference over the machine is adjusted by throttling in the valve. In this simple and cheap device however, a very high throttling loss occurs especially with low load.
In case of a hydro engine connected to the high pressure reservoir use can be made of an adjustable engine in which the oil volume taken in per revolution can be varied. With low load of the engine the control sets a small volume as a result of which a very small quantity of energy and a small drive torque is supplied per revolution. In this way a low loss adjustment to the capacity to be supplied can be realised. In hydro engines which cannot be adjusted and in particular with hydro cylinders, however, this way of controlling is impossible.
A control with low loss can also be achieved by transforming high pressure into low pressure with the help of an analogue pressure transformer consisting of an adjustable hydro pump driven by an adjustable hydro engine. With low load of the hydraulic machine the hydro engine is adjusted here to a small volume and the pump to a high volume per revolution. As a result a small volume of high pressure is supplied to the engine from the high pressure reservoir and converted or transformed to a much larger volume with low pressure which is supplied from the pump to the machine. Controlling with such an analogue pressure transformer or simplified varieties thereof such as the integrated analogue pressure transformer described in patent application PCT/NL97/00084 dated Feb. 24, 1997 gives small losses but is also relatively costly because the hydro engine/pump combination will always be a costly mechanical precision part of the device.
Known as well is the control of capacity and flow with the help of quick-working valves which work intermittently and during adjustable short periods of time admit the high pressure medium to the machine. In this way for instance a quick-working valve can supply oil intermittently to a lift cylinder with which a load can be lifted. When the valve is connected to a pressure reservoir of for instance 400 bar and the lift cylinder is loaded with a small load, which for instance causes a static pressure of 50 bar in the cylinder, the fluid will under influence of a large pressure difference of 400-50 bar flow in very fast when the valve opens. In a valve of small dimensions an unwanted throttle loss occurs here. In a valve of large dimensions the oil volume in question in the cylinder will almost immediately be compressed to the applied pressure of 400 bar. When the oil volume in question in the cylinder is for instance two liters, approximately 60 cc of oil will have to flow in as a result of the oil compression. When the valve closes now a relatively large quantity of energy is stored in the cylinder at that moment under the influence of which the load will start to move with large acceleration. As a result of this movement the oil in the cylinder will expand and the pressure will drop. At a certain moment the load will subsequently reach it maximum velocity in which the pressure has dropped again to 50 bar. After that the load will move under influence of its mass inertia in which the pressure in the cylinder drops further to below the pressure level of the low pressure reservoir to which the cylinder is connected via a non-return valve. The fluid will now flow from the low pressure reservoir to the cylinder via the non-return valve, which cylinder will decelerate relatively slowly under influence of its own weight until the velocity is zero and reverses sign and the load will slowly come down until the pressure in the cylinder has reached the level of 50 bar again. The load is now lifted over a length which as a rule will be much too large for the fine-tuned control which is nearly always required.
In order to lift the load in smaller steps much less oil has to be supplied from the quick switching valve per opening period. To that end said open period has to be reduced to such extreme short values that this cannot be obtained anymore or only obtained with much difficulty with the existing large dimensioned valves. In addition the valve then has to close at a moment on which the velocity of the fluid through the valve is very large, so that in order to prevent substantial loss of flow an extreme high closing speed is required. In addition a cavitation problem arises because the sudden breaking off of the fast flowing flow of fluid creates a vacuum directly behind the valve, as a result of which gas bulbs are formed in the fluid. At admitting high pressure again, these gas bulbs will implode at high speed, which leads to loss of energy and wear of material as a result of fatigue.
Although such known controls in principle are simple and cheap, in many cases, especially when used with machines with a large oil volume in question such as hydraulic cylinders, they are nonetheless accompanied by losses that are too large and they require quick working valves which according to the state of the art often do not match the requirements made to a satisfactory and fine-tuned control with a sufficiently low price in combination with low losses.
The problems mentioned and the losses that occur in the control with the help of fast working valves are prevented or reduced by the device according to the invention.
In this device an intermediate mass that is present is accelerated by the high pressure admitted during the open period of the quick switching valve, after which during the subsequent closed period of the quick switching valve said mass passes on its kinetic energy via the hydraulic medium to the oil volume in question at the entrance of the hydraulic machine and to the hydraulic machine itself.
According to a second aspect of the invention new digital quick switching valves are provided with which very short open or closed periods can be realized. This makes it possible to limit the quantity of oil per supply pulse and to dose it well, as a result of which a gradual pressure rise and gradual pressure drop of the pressure of the oil volume in question of the machine can be realized with little losses.
According to a third aspect of the invention the occurrence of cavitation in the control according to the invention is prevented by using an anti-cavitation accumulator (ACA), which will be elucidated in the following.
Said intermediate mass may consist of the mass of a fluid column between the quick switching valve and the machine, or of the mass of a mass moving or rotating along with this fluid column, for instance the rotation mass of a hydro engine accommodated in the supply pipe to the hydraulic machine, provided with a flywheel or not. To that end, in general a very simple type of hydro engine can suffice. The intermediate mass present in cooperation with the digital quick switching valve and the anti cavitation accumulator form a device for digital hydraulic pressure transformation (DHPT) which in the manner shown transforms the high pressure in the supply tank to a controllable average lower level at the location of the hydraulic machine. This device for digital hydraulic pressure transformation is, in comparison to the known means and devices, a compact and relatively cheap and low-loss means for capacity control and flow control for in particular hydraulic machines with a large volume in question of fluid such as hydraulic cylinders.
In order to realise such a device for digital hydraulic pressure transformation or xe2x80x9cDHPT devicexe2x80x9d, hydraulic valves and switches are required which work rapidly and which:
can switch a flow of fluid in a low loss manner;
are relatively cheap and
can be operated with control signals of little energy.
In addition is desirable that means are present to prevent cavitation.
The necessary valves, switches and anti-cavitation means are part of the device for digital hydraulic pressure transformation (DHPT) according to the invention.
In the simplest case the moving intermediate mass is the fluid mass in the intermediate pipe between the quick switching valve and the hydraulic machine. With a pipe length of for instance 4 meters and an cross-sectional surface of 1 cm2 that mass is 400xc3x971xc3x970.8 gram mass or 0.32 kilogram mass. With a sudden allowed pressure of for instance 300 bar the average acceleration of this fluid mass is approximately equal to 10000 m/sec2. After 1 and 2 milliseconds, respectively, the average fluid velocity in the pipe concerned is 10 and 20 meters per second. In order to limit the speed, bearing flow losses in mind, to acceptable values, the quick switching valve concerned therefore has to close the high pressure connection after about 2 to 3 milliseconds. This requires unusual high speed of switching which nonetheless can be obtained with the switches and the digital valves according to the invention. In addition, according to the invention it is possible to artificially increase the mass in question by incorporating a hydro engine in the supply pipe which engine is or is not provided with a flywheel. As a result of such a provision the fluid acceleration can be lowered with for instance a factor ten as a result of which the switching speed of the valve in question can be lowered and/or the energy of the supply pulse be reduced in favour of a more fine-dosed control of the average flow velocity to the hydraulic machine.
A good working of the device according to the invention subsequently requires that the danger of cavitation is prevented. In the device according to the invention this danger of cavitation is averted with the help of a so-called xe2x80x9cACAxe2x80x9d, an xe2x80x9canti-cavitation accumulatorxe2x80x9d. This accumulator is characterized in that the pressure generated by the accumulator at the fluid exit, cannot exceed a certain maximum and in that a pre-pressure is equal at the most to the lowest system pressure on the spot and in that the support of the accumulator membrane, in case of membrane accumulator, is at the gas side of the accumulator. As a result of these characteristics the accumulator membrane does not start to move until the fluid pressure at the fluid side of the membrane becomes lower than the lowest system pressure. This fluid side is connected to the space in which cavitation may occur, the cavitation being prevented because the fast expansion of the gas volume prevents the fluid pressure to become lower than the critical value of about 0.4-0.6 bar absolute. The ACA according to the invention in fact functions as a permanently present artificial gas bulb with a certain minimum volume and a certain maximum gas pressure, which at strong reduction of pressure in the fluid expands and as a result prevents too strong pressure reductions in the fluid pressure medium. Possible too quick a reduction of this artificial gas bulb at the rising again of the pressure can be prevented by throttling the gas or fluid flow from or to the ACA, respectively, in a known manner with help of for instance a non-return valve bridged by a restriction.